The subject matter disclosed herein relates to differential phase contrast imaging (DPCI), in particular, to methods and apparatus for optimizing grating alignment and grating dimensions to produce maximum phase contrast.
Phase contrast imaging (PCI) has emerged over the last several years as a useful imaging technique capable of probing phase characteristics of an object as complimentary information to its conventional absorption properties. Although, to date, several PCI techniques have been explored, some effort has been made to develop a grating-based DPCI technique that enables the use of a conventional broadband X-ray source. Conventional medical X-ray imaging devices rely on material absorption properties to provide information about an object's interior structure. While good contrast between strongly (hard) and weakly (soft) attenuating materials can be achieved, soft tissue differentiation can be difficult due to low relative contrast. For example, the low-contrast soft tissue materials including, but not limited to, vessels, cartilage, lung, and breast tissue, provide poor contrast in comparison to highly attenuating bone structures. The problem with soft-tissue imaging may be addressed by interferometric X-ray imaging devices, which utilize the wave nature of X-ray radiation. In addition to conventional absorption, such devices measure the phase shift experienced by an X-ray beam traversing the imaged object. The significantly larger atomic cross section of phase shift in comparison to absorption creates the potential for better sensitivity to material differentiation.
Several PCI imaging techniques may prove useful, including an interferometer technique, a diffraction-enhanced imaging technique, and a free-space propagation technique. Various difficulties associated with these techniques, such as the requirement of a synchrotron or micro-focus X-ray source, high sensitivity to mechanical instability, and large propagation distances, impose practical limitations on the development of clinically useful systems. Grating-based systems, such as Talbot-Lau PCI, may make possible an interferometer-based PCI system using a broadband X-ray source. Such a system takes advantage of the Talbot self-imaging interferometric effect to detect local phase shifts in the imaged object. Also, the geometrical alignment of the gratings may provide another technique for PCI system optimization.
An exemplary DPCI system may be assembled as shown in FIG. 1. The DPCI system 100 may include a polychromatic X-ray source 102 aimed at a beam shaping assembly that includes a filter F and a collimator C. Other assembled components of the DPCI system 100 placed in the path of the shaped x-ray beam include a partially absorbing source grating G0, an interferometric phase grating G1, a partially absorbing grating G2 (absorption grating), and an X-ray detector D which captures a radiographic image of an object 105. As illustrated, the object 105 is disposed between the source grating G0 and the phase grating G1. The openings s in source grating G0 each may have a period p0 of about tens of micrometers, which create partially coherent X-ray radiation 104 entering phase grating G1. Phase grating G1 may have a period p1 of about a few micrometers, which is less than p0, and generates a self-imaging interference pattern 106 which is further modulated by absorption grating G2, having a period p2 smaller than p1, onto an X-ray detector D. The gratings themselves may be made using well known manufacturing methods. The dimensions of the gratings, such as the periods (p0, p1, p2), or pitch, are fairly well controllable and may be manufactured according to desired design points. The imaging performance of such a system depends on grating dimensions, the geometry of the PCI setup (i.e., the relative position of the X-ray source, gratings (e.g., distances L and d), object, and X-ray receptor), X-ray beam quality, and detector properties.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.